This invention relates to an electrical power supply system, and more particularly to a low voltage power supply system for use in an application such as an electric blanket or the like.
An electric blanket, heating pad or throw typically includes internal resistive heating elements that function to emit heat when supplied with electrical power. Typically, the power supplied to the internal resistive heating elements is 120 volt AC electrical power, obtained from a conventional high voltage power source such as a wall outlet. While this type of power supply and heating arrangement functions satisfactorily, there are certain drawbacks associated with the supply of high voltage electrical power to the resistive heating elements incorporated in the blanket, heating pad or throw due to exposure of the user to the heating elements to which the high voltage power is supplied.
It is an object of the present invention to provide a power supply that converts high voltage input power to low voltage output power. It is a further object of the invention to provide such a power supply that is safe and reliable, and is capable of being used in an application such as an electric blanket, heating pad or throw, to avoid the drawbacks associated with use of high voltage electrical power in an application of this type. It is a further object of the invention to provide such a power supply in which the high voltage power input is isolated from the low voltage power output, to prevent the application of high voltage power to the load. Another object of the invention is to provide a power supply which automatically discontinues operation, or which prevents operation at start-up, in the event the load is disconnected from the power supply. Yet another object of the invention is to provide such a power supply which can be manufactured economically yet which is capable of efficiently and effectively providing a safe, reliable and controllable low voltage power output from a high voltage power input. Yet another object of the invention is to provide such a power supply capable of receiving and supplying power to a low voltage controller for regulating the output of the power supply.
In accordance with the present invention, a power supply, such as for use in an application such as an electric blanket, heating pad or throw, includes a power input arrangement adapted to receive power having a voltage exceeding a low voltage threshold. The power supply includes an isolation-type power conversion device, such as a transformer, that includes an input and an isolated output. The input of the power conversion device is interconnected with the power input arrangement, so that high voltage power is supplied to the input of the power conversion device. A switching arrangement is interconnected with the power conversion device for providing intermittent operation of the power conversion device, to provide output power to the isolated power output of the power conversion device. A power output arrangement is interconnected with the isolated output of the power conversation device. The intermittent operation of the power conversion device by the switching arrangement is operable to supply output power to the power output arrangement through the isolated output of the power conversion device, having a voltage below the low voltage threshold.
The power supply generally consists of a primary side, which is adapted to receive high voltage input power, and a secondary side isolated from the primary side via the isolation-type power conversion device, which is operable to output low voltage power. The isolation-type power conversion device may be in the form of a transformer that converts the high voltage input power to low voltage output power. The switching arrangement controls the duty cycle of the transformer, to control the power output of the transformer.
The output of the power supply is variable, according to the power demands of the load to which the power supply is connected. In the case of an electric blanket, heating pad or throw, the power supply includes an adjustable controller having settings that represent the amount of heat desired to be output by the resistive heating element, and the controller is operable to vary the power output of the power supply to control the power output so as to supply the desired heat output of the heating element. The controller is preferably interconnected with the secondary side of the power supply, to supply power to the controller. Outputs from the controller are supplied to an isolated feedback circuit associated with the secondary side of the power supply, and interact with an isolated and associated primary feedback circuit of the primary side of the power supply, e.g. via an IRED LED arrangement incorporated in the feedback circuits of the primary and secondary sides of the power supply. The switching arrangement is in the form of a power switch driven by a power switch driver interconnected with the feedback circuit of the primary side of the power supply. The feedback circuit of the primary side provides control of the power switch to control the low voltage power output of the transformer.
The primary side of the power supply includes a fundamental frequency oscillator which provides an output that is controlled by a network that varies the duty cycle of the fundamental frequency oscillator in response to inputs from the controller through the isolated primary and secondary feedback circuits, to control operation of the power switch and thereby the power output of the transformer. The primary side of the power supply further includes a burst logic circuit that provides an intermittent high frequency output that overrides the output of the fundamental frequency oscillator when the controller is turned off, to provide short bursts of enable power to the controller from the isolated output of the transformer. In this manner, low voltage power is continuously supplied to the controller from the transformer when the controller is off to enable operation of the controller. When the controller is turned on, the burst logic circuit is disabled and the output of the fundamental frequency oscillator is used to control the power output of the transformer.
The primary side of the power supply includes a current sensing arrangement which detects the presence of the load, and latches the primary side of the power supply to remain on when the load is present so as to enable operation of the power supply.
The components of the power supply function to convert high voltage input power to low voltage output power and to isolate the low voltage secondary side of the power supply from the high voltage primary side of the power supply. The invention thus provides a controllable source of low voltage output power for any type of load capable of operating on low voltage electrical power, including but not limited to use with an electric blanket, heating pad or the like.
The invention contemplates a power supply for use in any type of application, as well as a method of generating low voltage output power and an electric blanket having a low voltage power supply, substantially in accordance with the foregoing summary.